1. Field of the Invention
The present invention relates to method and apparatus for compression, method and apparatus for decompression, compression/decompression system, and record medium, particularly to a method for compression and decompression a continuous analog signal or digital signal.
2. Description of the Related Art
Conventionally, when transmitting and storing a signal such as a picture signal and an aural signal that has a large amount of information, a signal has been compressed and expanded in order to reduce an amount of transmitted information, increase a storing time of a storage medium, and so on. Generally, when an analog signal is compressed, an analog signal is initially sampled according to a predetermined sampling frequency and is digitized, and compression is performed on the obtained digital data.
For example, in the case of compression on a picture signal and an aural signal, a method is used in which compression is performed on a frequency region after original data is processed using a conversion filter on a time base-frequency axis such as DCT (Discrete-Cosine-Transform). DPCM (Differential Pulse Code Modulation), which is frequently used as a method of compression an aural signal for a telephone line, is used with the same intention. Additionally, the DPCM compression is a method for coding a difference of adjacent sample values when a waveform is sampled.
Further, as a method for performing time/frequency conversion, a method using a sub-band filter and MDCT (Modified Discrete Cosine Transform) is also available. As a coding method using such a method, MPEG (Moving Picture Image Coding Experts Group) audio is applicable.
Further, the most widely used picture compression system is generally known as the MPEG standard.
Data compressed by the above compression method is basically expanded according to reversed operations of the same compression method.
Namely, after compressed digital data is converted from a signal of a frequency region to a signal of a time region by frequency/time conversion, predetermined decompression operations are carried out to reproduce original digital data. And then, the original data obtained thus is subjected to digital-analog conversion if necessary and is outputted as an analog signal.
However, in the above conventional compression and decompression method, a signal on a time base is converted to a signal on a frequency axis before compression. Hence, operations such as time/frequency conversion for compression and frequency/time conversion for expansion are necessary. Therefore, the operations are complicated and the configuration for realizing the operations becomes extremely complicated. This problem has caused not only a longer processing time for compression and expansion but also difficulty in achieving a smaller device.
Moreover, generally in the case of compression and expansion of data, it is important to consider how to improve the quality of reproduced data while improving its compressibility. However, in the above conventional compression and decompression method, when a compressibility of a picture signal and an aural signal is increased, an image and voice reproduced by decompression compressed data are degraded in quality. In contrast, when importance is placed on the quality of a reproduced image and reproduced voice, a picture signal and an aural signal decreases in compressibility. Therefore, it has been extremely difficult to achieve both of an increased compressibility and improved quality of reproduced data.
The present invention is devised to solve the above problems and aims to simplify the compression and decompression operations for a signal so as to shorten a processing time and to simplify the configuration for realizing the operations.
Also, another object of the present invention is to provide a new compression and decompression method for increasing compressibility and improving quality of reproduced data.
Furthermore, another object of the present invention is to more readily perform compression and expansion without using a table.
In order to solve the above-mentioned problems, on a compression side of the first invention, a signal to be compressed is sampled at a time interval of a point where a differential absolute value is at a predetermined value or smaller, a pair of discrete amplitude data on each sample point and timing data indicative of a time interval between sample points is obtained as compressed data.
Here, a signal to be compressed may be oversampled, and the oversampled data may be sampled at a time interval of a point where a differential absolute value is at a predetermined value or smaller. Further, on the oversampled data, the operation for generating average value data of successive sample values may be further performed.
Moreover, on an expansion side of the first invention, regarding compressed data composed of a pair of amplitude data on predetermined sample points extracted from a signal to be compressed and timing data indicative of a time interval between sample points, amplitude data on successive sample points and timing data therebetween are used to obtain interpolation data for interpolating pieces of amplitude data having a time interval indicated by the timing data. Thus, expansion data is obtained.
Here, a sampling function obtained from two pieces of amplitude data on two successive sample points and timing data therebetween may be used to obtain interpolation data for interpolating the two pieces of amplitude data.
According to the first invention configured thus, when a signal is compressed on a time base, the operation can be performed on a time base without performing time/frequency conversion on a frequency axis. Besides, when data compressed thus is expanded as well, the operation can be performed on a time base. Therefore, it is possible to simplify compression and expansion, to shorten an operating time, and to simplify the configuration for the operations. Also, when compressed data is transmitted from the compression side and is reproduced on the expansion side, a simple interpolating operation on a time base can sequentially process and reproduce compressed data inputted to the expansion side, thereby realizing real-time operations.
Besides, in the present embodiment, only data on sample points of sampling points can be obtained as compressed data, thereby achieving high compressibility. The sample points are equivalent to inflected points in a signal to be compressed, and the sample points include all minimum points required for reproducing original data by an interpolating operation on the expansion side. Therefore, it is possible to obtain high-quality reproduced data with higher reproducibility of original data.
Further, on the compression side of the second invention, digital data of a basic waveform corresponding to values of inputted n pieces of discrete data is synthesized by oversampling and a moving average operation or a convoluting operation so as to obtain digital interpolation values for the discrete data. Thereafter, the digital interpolation values are sampled at a time interval of a point having a minimum differential absolute value, and a pair of discrete amplitude data on sample points and timing data indicative of a time interval between sample points is obtained as compressed data.
Moreover, on the expansion side of the second invention, by using amplitude data and timing data that are included in compressed data, based on two pieces of amplitude data on two successive sample points and timing data therebetween, interpolation data for interpolating the two pieces of amplitude data is determined so as to obtain expansion data.
According to the second invention configured thus, when data is compressed on a time base, the operation can be performed on a time base without performing time/frequency conversion on a frequency axis. Besides, when data compressed thus is expanded as well, the operation can be performed on a time base. Therefore, it is possible to simplify compression and expansion, to shorten an operating time, and to simplify the configuration for the operations. Also, when compressed data is transmitted from the compression side and is reproduced on the expansion side as well, a simple interpolating operation on a time base can sequentially process and reproduce compressed data inputted to the expansion side, thereby realizing real-time operations.
Besides, only data on sample points of sampling points can be obtained as compressed data, thereby achieving high compressibility. The sample points are equivalent to inflected points in a signal to be compressed, and the sample points include all minimum points required for reproducing original data by an interpolating operation on the expansion side. Therefore, it is possible to obtain high-quality reproduced data with higher reproducibility of original data.
Additionally, according to the second invention, when an interpolation value is obtained by performing oversampling and convolution on digital data, only values of a limited number of pieces of discrete data need to be considered for determining a certain interpolation value, thereby preventing a censoring error. Thus, an interpolation value can be obtained accurately. Therefore, regarding data reproduced on the expansion side when compression is performed using the interpolation value, it is possible to improve reproducibility to original data before compression.
Besides, on the compression side of the third invention, inputted digital data is differentiated, a point where a differential value changes in polarity is detected as a sample point, and digital data rounded by a predetermined value is obtained as discrete compressed amplitude data on sample points. A pair of compressed amplitude difference data, which is obtained by computing a difference between pieces of the compressed amplitude data, and timing data indicative of a time interval between sample points is obtained as compressed data.
Further, on the expansion side of the third invention, the compressed amplitude difference data, which is oversampled by even-numbered times, is subjected to multiple integral, and a moving average operation is performed on the integral value. A moving average value obtained thus and timing data is used to obtain square-law interpolation data, which interpolates pieces of amplitude data on sample points having a time interval indicated by the timing data, as expansion data.
In another aspect of the third invention, on the compression side, inputted digital data is rounded by a first value, the digital data rounded by the first value is differentiated and a point where a differential value changes in polarity is detected as a sample point. Digital data rounded by a second value, which is larger than the first value, is obtained as discrete compressed amplitude data on sample points.
In another aspect of the third invention, on the compression side, the compressed amplitude difference data and the timing data are converted to variable-length block data.
In another aspect of the third invention, on the expansion side, the compressed amplitude difference data oversampled by even-numbered times is reversed in sign at an intermediate position of each section between sample points, the section being indicated by the timing data. Data strings obtained thus are subjected to multiple integral.
In another aspect of the third invention, on the expansion side, multiple integral and a moving average operation are performed in each section between sample points on the compressed amplitude difference data oversampled by even-numbered times.
According to the third invention configured thus, when a signal on a time base is compressed, the operation can be performed on a time base without performing time/frequency conversion on a frequency axis. Besides, when data compressed thus is expanded as well, the operation can be performed on a time base. Therefore, it is possible to simplify compression and expansion, to shorten an operating time, and to simplify the configuration for the operations. Also, during expansion, a simple square-law interpolating operation on a time base can sequentially process and reproduce inputted compressed data without using table information, thereby realizing real-time operations.
Furthermore, according to the third invention, compressed data can be generated from only few pieces of discrete data that include an amplitude data value on a sample point where digital data has a differential value changing in polarity and a timing data value indicative of a time interval of sample points. Besides, since amplitude data on sample points is rounded by a predetermined value, it is possible to shorten a data length of amplitude data by several bits per word, thereby largely reducing an amount of data. Additionally, in the third invention, rounded amplitude data is not used as compressed data directly but difference data is further determined as compressed data. Hence, it is possible to further reduce the number of bits required for compressed data, thereby reducing an amount of data.
Moreover, according to other characteristics of the third invention, compressed amplitude difference data and timing data that are obtained thus are encoded to variable-length block data as final compressed data. Thus, compressibility can be further increased.
Besides, according to the third invention, an inflection point existing in a signal to be compressed is detected as a sample point, and compressed data includes all minimum points required for reproducing original data by an interpolating operation on the expansion side. Therefore, it is possible to increase reproducibility of original data, thereby obtaining high-quality reproduced data.
Additionally, according to another characteristic of the third invention, digital data rounded by a suitable value is differentiated to detect a sample point. Hence, it is possible to prevent positions of noise components and unnecessary signal components from being detected as sample points, thereby positively detecting only correct positions as sample points. Therefore, as for expansion data reproduced on the expansion side, it is possible to improve reproducibility of original data before compression.
Further, according to another characteristic of the third invention, compressed amplitude difference data oversampled by even-numbered times is reversed in sign at an intermediate position of each section between sample points. Hence, when multiple integral and a moving average operation are performed on data strings reversed in sign, it is possible to compensate for a rounding error on the compression side and to reproduce a digital waveform having more smoothly changing amplitude values.
Besides, according to another characteristic of the third invention, on the expansion side, multiple integral is performed in each section between sample points. Thus, it is possible to eliminate an accumulative error caused by integral, thereby reproducing a digital waveform more accurately.